You might get away with 320 under solid colors but for metallic I wouldn't use anything coarser than 600.
As far as paint job delaminating-some people wait too long before topcoating/clearing.

I do it every day...320 DA ,2-3 coats of base then clear..I've never seen anybody sand a car with 600 .....even If I'm blending I'll 320 da where the color is then 600 DA the blend area and the rest of the panel....I never use a sealer just a few coats of base on the primer spots before I start painting....Thats what I've used for my production work for the past 20 yrs..If your getting scratches showing through with 320 DA your not getting enough paint on, 2-3 coats of any base is plenty ...
Now,hand sanding and wet sanding is different. When wet or dry sanding the coarsest you can use would be 400 before you can paint....The only time I use any kind of sealer is when I'm doing a complete paint job thats been completely primed and blocked(by hand) and then its an epoxy primer reduced to a paints viscosity..

The Following User Says Thank You to deadbodyman For This Useful Post: